1. Field of the Invention
The present invention relates to a semiconductor device which is provided with a circuit including a semiconductor element such as a transistor. For example, the present invention relates to a power device which is mounted on a power supply circuit; a semiconductor integrated circuit including a memory, a thyristor, a converter, an image sensor, or the like; an electro-optical device typified by a liquid crystal display panel; a light-emitting display device including a light-emitting element; and an electronic device including any of the above as a component.
In this specification, a semiconductor device refers to all types of devices which can function by utilizing semiconductor characteristics; an electro-optical device, a light-emitting display device, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
Many transistors formed over a glass substrate or the like are manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although transistors including amorphous silicon have low field-effect mobility, they can be formed over a larger glass substrate. On the other hand, transistors including polycrystalline silicon have high field-effect mobility, but they are not suitable for being formed over a larger glass substrate.
Other than transistors formed using silicon, a technique in which a transistor is formed using an oxide semiconductor and applied to an electronic device or an optical device has attracted attention. For example, a technique of manufacturing a transistor by using zinc oxide or In—Ga—Zn—O-based oxide as oxide semiconductor, and of using the transistor as a switching element of a pixel of a display device and the like is disclosed in Patent Document 1 and Patent Document 2.
A transistor including an oxide semiconductor film is expected to considerably improve performance of a display device because such a transistor has higher field-effect mobility than a transistor including amorphous silicon.